Maximal angular correlation in γ−γ\gamma-\gamma coincidences: a quantitative study

The measurement of the angular distribution of maximally correlated annihilation gamma rays radiated in coincidence, like those emitted from a $^{22}\mathrm{Na}$ source, is a classic experiment that is nowadays ordinarily performed in Nuclear Physics laboratory classes. For the first time we present an analytic expression for such angular distribution, which can be easily tested and confronted with the laboratory measurements.Comment: 12 pages, 3 figure

Dilatonic black holes in superstring gravity

We solve the dilaton field equation in the background of a spherically symmetric black hole in type II superstring theory with $\alpha'^3$ corrections in arbitrary $d$ spacetime dimensions. We then apply this result to obtain a spherically symmetric black hole solution with $\alpha'^3$ corrections in superstring theory compactified on a torus, coupled to such dilaton. For this black hole we obtain its mass, entropy, temperature, specific heat, and free energy.Comment: V2: some small changes in the presentation; references and discussion added; results unchanged. Published version. arXiv admin note: text overlap with arXiv:0912.305

Scattering of spherically symmetric d-dimensional alpha'-corrected black holes in string theory

We study scattering of minimally coupled massless scalar fields by non-extremal spherically symmetric black holes in d dimensions with string-theoretical alpha' corrections. We then obtain a formula for the low frequency absorption cross section for every black hole of this kind, which we apply to known black hole solutions. We compare the alpha' corrections for the absorption cross section with those for the black hole entropy, obtained through Wald's formula, in each case concluding that these corrections are different. We find a general covariant formula for the absorption cross section including alpha' corrections, in terms of the horizon area and temperature.Comment: 27 pages. v2: some clarifications; discussion on the scalar field equation improved; some parts moved from the main text to two appendices. v3: discussion on dependence under field redefinitions extended; new section with scheme-independent formulae for entropy and cross sectio

Absorption of scalars by extremal black holes in string theory

We show that the low frequency absorption cross section of minimally coupled test massless scalar fields by extremal spherically symmetric black holes in d dimensions is equal to the horizon area, even in the presence of string--theoretical $\alpha'$ corrections. Classically one has the relation $\sigma = 4 G S$ between that absorption cross section and the black hole entropy. By comparing in each case the values of the horizon area and Wald's entropy, we discuss the validity of such relation in the presence of higher derivative corrections for extremal black holes in many different contexts: in the presence of electric and magnetic charges, for nonsupersymmetric and supersymmetric black holes, in d=4 and d=5 dimensions. The examples we consider seem to indicate that this relation is not verified in the presence of $\alpha'$ corrections in general, although being valid in some specific cases (electrically charged maximally supersymmetric black holes in d=5). We argue that the relation $\sigma = 4 G S$ should in general be valid for the absorption cross section of scalar fields which, although being independent from the black hole solution, have their origin from string theory, and therefore are not minimally coupled.Comment: Published version with minor corrections. 23 page

Perturbation theory and stability analysis for string-corrected black holes in arbitrary dimensions

We develop the perturbation theory for R^2 string-corrected black hole solutions in d dimensions. After having obtained the master equation and the alpha'-corrected potential under tensorial perturbations of the metric, we study the stability of the Callan, Myers and Perry solution under these perturbations.Comment: Presented at 28th Spanish Relativity Meeting (ERE05): A Century of Relativity Physics, Oviedo, Asturias, Spain, 6-10 Sep 200

A high-precision liDAR-based method for surveying and classifying coastal notches

Formation of notches is an important process in the erosion of seaside cliffs. Monitoring of coastal notch erosion rate and processes has become a prime research focus for many coastal geomorphologists. Observation of notch erosion rate considers a number of characteristics, including cliff collapse risk, distinction of historical sea levels, and recognition of ongoing erosional mechanisms. This study presents new approaches for surveying and classifying marine notches based on a high-precision light detection and ranging (LiDAR)-based experiment performed on a small region of a coastal cliff in southern Portugal. A terrestrial LiDAR scanner was used to measure geometrical parameters and surface roughness of selected notches, enabling their classification according to shape and origin. The implemented methodology proved to be a highly effective tool for providing an unbiased analysis of marine morphodynamic processes acting on the seaside cliffs. In the analyzed population of voids carved into Miocene calcarenites in a coastal cliff section, two types of notch morphology were distinguished, namely U-shaped and V-shaped. The method presented here provides valuable data for landscape evaluation, sea-level changes, and any other types of analyses that rely on the accurate interpretation of cliff morphological features.National Science Centre [UMO-2015/17/D/ST10/02191